There continues to be a need for inexpensive noninvasive alcohol sensors which determine the concentration of alcohol in various gas mixtures. Development of such a sensor is of importance since determining the alcohol concentration is necessary in various applications, for example detecting the blood alcohol value by determining ethanol in expired air or on the skin, in the membrane biotechnology sector, for example in fermenter processes, or in room air monitoring, for example for workplace safety. There are presently no sensors which have sufficiently long-term stability and which are inexpensive.
Determining the alcohol concentration in expired air for monitoring compliance with the blood alcohol limits is important particularly in the area of testing one's ability to drive in traffic. For this application it is necessary to develop inexpensive alcohol sensors in order to provide versatile expedient hand-operated instruments.
In the field of asthma diagnosis by means of nitrogen oxide detection in expired air, elimination of interfering effects is of great importance, for example eliminating effects of other gases which are present in high concentration. The ethanol concentration in expired air can rise up to 1500 ppm. Using an alcohol gas sensor, the concentration of ethanol can thus be determined in parallel, and thus the quality of the actual nitrogen oxide detection checked, and if appropriate, improved.
In addition to determining the alcohol concentration in expired air, alcohol content following alcohol consumption can also be detected on the skin. This provides the possibility for continuously measuring alcohol content without the active cooperation of the person being tested, and hence is ideal for monitoring persons who are, for example, in emergency medical situations.
Alcohol concentration measurements in the biotechnology sector are important for processes in which methanol is used as substrate in fermenter processes. Maintaining the methanol concentration precisely within a defined optimum range is of great importance for carrying out the fermentation and maintaining the fermentation processes. Monitoring the alcohol concentration in ambient air, for example in the workplace, is of great importance for ensuring workplace safety and compliance with MAC values. These are, with respect to ethanol, 1000 ppm, and, with respect to methanol, 200 ppm. Here also, the use of an inexpensive alcohol sensor or even of a personal alcohol alarm, is desirable.
Some known commercial alcohol sensors function primarily according to the resistive principle, where semiconductor materials are used as a gas-sensitive layer. A known gas-sensitive material for such sensors is tin oxide. Gallium oxide as a semiconductor material also has a very good sensitivity, and compared with tin oxide has a higher thermal long-term stability. By combining various filter layers over the gas-sensitive layers, highly selective sensors for detecting ethanol have been developed. Known materials for filter layers are, for example, silicon dioxide or aluminum oxide. These developments have been able to suppress cross-sensitivities to interfering gases. However, a disadvantage of semiconductor alcohol sensors is the power requirement for the obligatory sensor heating, and for this reason, sensor variants for use as modular elements or in battery-operated instruments, for example a cell telephone, are not currently possible.
In addition to the foregoing, electrochemical alcohol sensors are known which are also used as hand-operated instruments for measuring alcohol in the breath. A disadvantage of these sensors is that they have only a limited life and are costly.
In the area of optical methods of measurement, there are infrared sensors for determining alcohol in expired air, or gas sensors which are based on the principle of cataluminescence. Infrared detection using bandpass filters frequently shows a high cross-sensitivity to various hydrocarbons and gases, in which case selective detection of alcohols under actual conditions cannot be guaranteed. Although cataluminescence-based alcohol sensors can differentiate between various alcohols, for example between ethanol and butanol, they cannot differentiate between alcohols and ketones. Furthermore, these optical methods of measurement are also very expensive.